HR: 1330h
AN: V12D-0617    [PDF]
TI: Coherent Time-Dependent Variation of $^{87}$Sr/$^{86}$Sr in Clinopyroxene From the Alban Hills Volcanic District (Central Italy): Clues to Source Evolution
AU: Karner, D B
EM: daniel.karner@sonoma.edu
AF: Dept. of Geology, Sonoma State University, 1801 East Cotati Avenue, Rohnert Park, CA 94985 United States
AU: Christensen, J N
EM: jnchristensen@lbl.gov
AF: Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Bldg 70A, 4418, Berkeley, CA 94720-8179 United States
AU: * Freda, C
EM: freda@ingv.it
AF: Dept. of Seismology and Tectonophysics, Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AU: Gaeta, M
EM: mario.gaeta@uniroma1.it
AF: Dept. of Earth Sciences, University "La Sapienza", P.le Aldo Moro 5, Rome, 00185 Italy
AU: Marra, F
EM: marra@ingv.it
AF: Dept. of Seismology and Tectonophysics, Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AU: Scarlato, P
EM: scarlato@ingv.it
AF: Dept. of Seismology and Tectonophysics, Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AB: It is unclear whether the Alban Hills volcano near Rome is extinct or whether it is inactive. To address this question we have been studying the chemical evolution of its eruptive products, where unaltered materials are found only as inclusions in phenocrysts. We measured the $^{87}$Sr/$^{86}$Sr of Sr in clinopyroxene phenocrysts from a complete suite of volcanic products (pyroclastic flows, lava flows, and granular ejecta) of the ultra-potassic Alban Hills Volcanic District, encompassing its entire eruptive history for the time span 608 - 35 ka. Each analyzed sample was dated by means of $^{40}$Ar/$^{39}$Ar geochronology, in order to investigate the evolution of the magmatic system. The $^{87}$Sr/$^{86}$Sr of the pyroxenes ranges from 0.7112 to 0.7085, and a correlation between eruption age and $^{87}$Sr/$^{86}$Sr is apparent. In particular, $^{87}$Sr/$^{86}$Sr shows a near-linear, time-dependent decrease from the oldest to youngest samples. An exception to this occurs at 70 ka, the age of the start of the latest eruptive cycle in the Albano Maar, whose pyroxenes have $^{87}$Sr/$^{86}$Sr below this long-term trend. Based on the long duration of the eruptive history and the large volume of erupted products, we interpret the general trend as representative of variation in the magma source, rather than assimilation during storage in crustal magma chambers. Partial melting of metasomatically-veined and fertilized mantle has been proposed to explain the origin of these ultra-potassic magmas. If correct, the decreasing $^{87}$Sr/$^{86}$Sr with time could reflect the progressive exhaustion of the metasomatic components (i.e. veins) in the mantle source region. In contrast, in the light of independent petrologic and geochronologic data, we hypothesize that the 70 ka anomaly may be linked to a renewal of the magma source.
DE: 1010 Chemical evolution
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting